What is the difference between a 357 and a 357 magnum?

Answer 1

.357 is the measurement of the bore of a gun.

.357 Magnum is the name for a particular rimmed cartridge that uses .357 bullets.

There are other cartridges using the same size bullet, but they are named differently because they are not interchangeable with .357 Magnum due to different case dimensions.

People often call .357 Magnum just ".357" for short because it is the most common cartridge with this number in the name. It's the same as 9mm Luger being shortened to just "9mm" because it's the most common of the nines.